This invention relates to compositions classified in the art of chemistry as dialkyl peroxydicarbonates, more specifically to new and novel thermally-stabilized initiator compositions comprising dialkyl peroxydicarbonates stabilized by the addition thereto of a stabilizing effective amount of at least one stabilizing compound selected from the group consisting of diesters of unsaturated dicarboxylic acids. The invention further relates to the use of such novel compositions as initiators of polymerization or cure of ethylenically unsaturated monomers, oligomers and polymers. The invention still further relates to the stabilization of dialkyl peroxydicarbonates during their manufacture by the inclusion of stabilizing effective amount of at least one stabilizing compound selected from the group consisting of non-hydrolyzable diesters of unsaturated dicarboxylic acids in the mixture of reactants from which a dialkyl peroxydicarbonate is to be prepared prior to, at the commencement of, or during the preparative reaction.